Scotland Yard officers failed to consult either the director of public prosecutions or the attorney general before invoking the Official Secrets Act to try to force the Guardian to reveal journalistic sources, it has been revealed.

Keir Starmer, the DPP, said he was approached for advice by the Metropolitan police professional standards squad only this afternoon, following leading articles in all Britain's major newspapers condemning police behaviour in pursuing the Guardian's sources for their revelations about the phone-hacking scandal. Prosecutions under the section of the Official Secrets Act concerned, Section 9 (2) of the 1989 legislation, can go ahead only with the specific consent of Starmer, who is an independent professional lawyer. Other sections of the act, concerned with espionage, require the consent of the attorney general, Dominic Grieve.

Grieve's office said he had not been consulted in advance.

Although police are not technically required to get the DPP's permission at an early stage of inquiries, Starmer told a media law conference he encouraged sensitive police investigations to be carried out from an early stage in close consultation with the DPP's lawyers from the Crown Prosecution Service.

Asked about the Guardian case, he said: "We have now been asked to advise on the Official Secrets Act … We are now engaged in providing advice."

Police want a court order to force Guardian reporters to reveal confidential sources for articles disclosing that the murdered Milly Dowler's phone was hacked on behalf of the News of the World. They claim that the reporter Amelia Hill may have "incited" a source to break the Official Secrets Act. At the Metropolitan police it was being maintained the decision to plunge its new commissioner, Bernard Hogan-Howe, into controversy was taken by relatively junior officers without his knowledge. The Metropolitan police said: "The decision to apply for the production order was an operational one made by the senior investigating officer (SIO) with the benefit of legal advice, as is normal practice. The matter was not referred to a more senior level."

Scotland Yard's legal director is Edward Solomons, and the head of the professional standards department, which is bringing the action against the Guardian and its reporters, is deputy assistant commissioner Mark Simmons. Both reported directly to Hogan-Howe, according to the Metropolitan police organisational chart.

Under Section 9 (2) of the 1989 act, prosecutions for leaks involving police, prisons and the justice system, normally through corrupt links with criminals, are allowed to take place with the consent of the DPP. This is less of a hurdle than imposed for the other sections of the act, which need a political go-ahead from the attorney general, who supervises the DPP. But it appears the Met did not take advice from any outsider before serving production orders on the Guardian and Hill.

An Old Bailey judge is to be asked on Friday to sign off on the orders, forcing the Guardian to hand over material identifying its sources for the Dowler revelation and for stories identifying former executives from the Murdoch media empire who had been arrested for questioning, including David Cameron's former PR chief, Andy Coulson, and the former News International chief executive Rebekah Brooks.

The police claim a detective on the official hacking inquiry, Operation Weeting, may have leaked details to the Guardian in "a gratuitous release of information that was not in the public interest". The Guardian says its stories revealed crucial information that was entirely in the public interest and that it has a duty to protect its sources. The Guardian has not paid police officers for its information.

In a statement on Monday night , the Met said: "The application for a production order against the Guardian newspaper and one of its reporters is part of an inquiry by officers from the MPS directorate of professional standards anti-corruption unit (DPS), not Operation Weeting ... The MPS cannot respond to the significant public and political concern regarding leaks from the police to any part of the media if we aren't robust in our investigations and make all attempts to obtain best evidence of the leaks."

Dunja Mijatovic, OSCE representative on freedom of the media, wrote to the foreign secretary, William Hague, saying she was greatly concerned about the potentially chilling effect on investigative journalism and media freedom of the Official Secrets Act moves: "The right of journalists to protect the identity of their confidential sources has been repeatedly declared a basic requirement for freedom of expression by the OSCE."

Keith Vaz, chairman of the Commons home affairs committee, wrote to Hogan-Howe asking "whether this marks a change in the Metropolitan police's approach to the behaviour of journalists who seek and receive confidential information in the public interest".

The DPP's own official guidance makes it clear that police should normally proceed against reporters with caution. "Investigation and prosecution of cases involving the leaking of confidential information to journalists can present especial difficulty ... the courts have demonstrated a reluctance to order disclosure of journalistic sources ... freedom of the press is regarded as fundamental to a free and democratic society.The ability of a journalist to protect a source of information is afforded significant protection by the law."

The second charge police say they are contemplating, is "misconduct in public office". The DPP's guidance also warns against unwise use of this charge: "Not every act of misconduct by a public official is capable of amounting to a criminal offence. There is a threshold and it is a high one."

At the Liberal Democrat conference, where delegates called for tougher sanctions against tabloid misbehaviour, deputy leader, Simon Hughes, said the Met's attack on the Guardian's sources was inappropriate. "That was responsible journalism," he said. He hoped the government would not allow the prosecution of such sources to proceed.

Mark Lewis, the solicitor representing the family of murder victim Milly Dowler, told delegates that the "official secrets" that the Dowler family were concerned about were not the leaks to the Guardian in 2011, but the secrets kept by the Met police between 2006 and 2011, which he said prevented the family – and thousands of other victims of hacking – from knowing they were victims at all.

Mark Stephens, at a legal conference in London, said later: "Someone in the Met should lose their pips for the suggestion that journalists should be criminalised."

Ivan Lewis, the shadow culture secretary, said: "It was brave investigative journalism that brought this matter of huge public concern to light, therefore the Metropolitan police and CPS need to take great care not to do anything that would undermine the ability of the press to shine a light on issues of public interest in the future.

"The attorney general and director of public prosecutions should be seriously questioning whether any action currently proposed would be compatible with these aims."

Lord Lester, the Liberal Democrat human rights lawyer, said that it seemed to be "an abuse of the Official Secrets Act" to use it to force journalists to hand over information unless "unless there's an overwhelming public interest to the contrary". He added: "I'm not aware of that overwhelming public interest in this case. I hope the Metropolitan police will reconsider."

Another prominent London media lawyer, Caroline Kean, said: " I think it's outrageous. They must be using it to prevent other leaks. It does seem extraordinarily draconian."

Jean-Yves Dupeux, a media lawyer from Paris, said he did not think the police's attempt to force the Guardian to reveal its sources would succeed in the European courts. "It's contrary to Article 10 of the human rights convention," he maintained. "Perhaps it might work in something very important like a terrorist or human trafficking case, but not [against] journalists for phone hacking."

• This article was amended on 20 and 22 September 2011 to clarify a quote from Mark Stephens and correct Dunja Mijatovic's job title.